1. Field of the Invention
This invention relates to a method for covering a substrate with a cylindrically shaped heat-shrinkable film.
2. Description of the Prior Art
There have heretofore been used cylindrically shaped heat-shrinkable films for the purpose of protection of various articles. A cylindrically shaped heat-shrinkable film is a film in a shape of cylinder which can shrink by heating, and it is a general practice to apply a cylindrically shaped heat-shrinkable film over a certain article followed by heating, whereby there is formed a covering in close contact along the surface contour of the article. Thus, surface covering is possible with the use of a cylindrical heat-shrinkable film only by heating and within a short period of time, and hence cylindrically shaped heat-shrinkable films have frequently been used for various packaging purposes. However, when a cylindrically shaped heat-shrinkable film is utilized for covering of an article for which a precision is required, a cylindrically shaped heat-shrinkable film chosen is required to be as flawless as possible. Nevertheless, the heat-shrinkable films perpared according to the methods of prior art suffer from lines of flection formed thereon, which will being about deleterious influences on covering of an article for which a percision is required. For example, in case of surface covering on an electrophotographic photosensitive material, the image characteristics at the line of flection are extremely worsened to give rise to disorders of image or bad cleaning performance.
Formation of a line of flection on a cylindrically shaped heat-shrinkable film of prior art is due to its production method, as explained now with reference to the drawing. FIG. 1 is a typical example of the production method of prior art. That is, a starting material resin 1 is fed into a hopper 6 and then melted by heating. The molten starting material resin progresses along the arrow 7 until it is extruded through an annular die portion 2. And, an inflating air is blown through a blow inlet for stretching of the extruded cylindrically shaped film. The staring material resin molten by heating is extruded through the annular die portion 2 and, while being wound up and drawn upwardly simultaneously with blowing of the inflating air to perform stretching operation, the cylindrically shaped heat-shrinkable film 8 is flexed through the roller 4 and wound up on the wind-up roller 5. Thus, the line of flection is formed in the course of winding up the fabricated cylindrically shaped heat-shrinkable film.
Even when using a cylindrically shaped heat-shrinkable film free from such a line of flection, it is not easy to have the film shrunk fitted to the contour of a substrate. That is, in the state prior to heat shrinkage, a cylindrically shaped heat-shrinkable film covers the surface of a substrate. But not all the portions of the cylindrically shaped heat-shrinkable film contacts the surface of the substrate, that is, some portions are contacted while other portions remain uncontacted. The heat-shrinkable film at the portions contacted with the substrate will permit a considerable amount of heat to be dissipated into the substrate on heating and therefore undergoes heat shrinking at a slow rate. On the other hand, the heat-shrinkable film at the portions not contacted with the substrate will difficultly release the heat applied toward the substrate and therefore heat shrinking occurs rapidly. As the result, there takes place partially irregularity in the extent of heat shrinkage, and therefore, the film thickness of the portions shrunk earlier increases while that of the portions shrunk later is reduced, whereby no uniform heat shrinkage can be accomplished.